1. Field of the Invention
This invention relates to EPROM memory devices and more particularly to enhancement mode EPROM and flash EEPROM devices and method of fabrication of such devices.
2. Description of Related Art
EPROM (Erasable Programmable Read Only Memory) FET semiconductor devices provide long term retention of charge to store data. The charge is stored on a floating gate electrode which is not electrically connected to a terminal. Instead, the charge is supplied to the floating gate electrode through an insulator. The data may be erased by exposure of the device to ultraviolet light to erase the stored data by discharging the floating gate electrode.
EEPROM (Electrically Erasable Programmable Read Only Memory) FET semiconductor devices can be erased by electrical means instead of exposure to ultraviolet light.
Flash EPROM devices are EEPROM FET devices with a double gate electrode structure including a floating gate electrode and a control gate electrode with a dielectric layer thickness of about 100 .ANG. thickness.
U.S. Pat. No. 5,198,380 (Harari); U.S. Pat. No. 4,816,883 (Baldi); U.S. Pat. No. 4,646,425 (Owens et al.) show methods of forming EPROM devices. However, these differ from the EEPROM of the invention.
U.S. Pat. No. 5,506,816 (Hirose et al.) shows a memory cell array having a compact word line arrangement. A subword line drives circuit has an NMOS transistor and a PMOS transistor, with the drain of the NMOS transistor and the source of the NMOS transistor connected to the word line. The source of the NMOS transistor and the drain of the PMOS transistor are connected to the receive a word line selection signal from a common source line. However, this differs form the EEPROM of the invention.
U.S. Pat. No. 5,506,803 (Jex) shows a memory device where the driver is a P-channel MOS having an N-well which is used program. However this differs from the EEPROM of the invention which uses a P-channel MOS to program and a N-channel MOS to write.
Problems which can occur with P-channel EEPROM devices include the fact that a P-channel EEPROM has higher Channel Hot Electron (CHE) injection current at lower gate voltage, and a P-channel EEPROM has serious drain disturbance and depletion issues.